The C Programming Language P8.5

#include <fcntl.h>
#define PERMS 0666

#define NULL 0
#define EOF (-1)
#define BUFSIZE 1024
#define OPEN_MAX 20/* max #files open at once */

typedef struct _iobuf {
int cnt; /* characters left */
char *ptr; /* next character position */
char *base; /* location of buffer */
int flag; /* mode of file access */
int fd; /* file descriptor */
} FILE;
extern FILE _iob[OPEN_MAX];
#define stdin (&_iob[0])
#define stdout (&_iob[1])
#define stderr (&_iob[2])

enum _flags {
_READ = 01, /* file open for reading */
_WRITE = 02, /* file open for writing */
_UNBUF = 04, /* file is ubuffered */
_EOF = 010, /* EOF has occured on this file */
_ERR = 020 /* error occurred on this file */
};
int _fillbuf(FILE *);
int _flushbuf(int, FILE *);
FILE *fopen(char *name, char *mode);
int fflush(FILE *);
int fclose(FILE *);
int fseek(FILE *fp, long offset, int origin);

#define feof(p) (((p)->flag&_EOF)!=0)
#define ferror(p) (((p)->flag&_ERR)!=0)
#define fileno(p) ((p)->fd)

#define getc(p) (--(p)->cnt>=0\
?(unsigned char) *(p)->ptr++:_fillbuf(p))

#define putc(x,p) (--(p)->cnt>=0\
?*(p)->ptr++=(x):_flushbuf((x),p))

#define getchar() getc(stdin)
#define putchar(x) putc((x),stdout)

FILE _iob[OPEN_MAX] = { { 0, (char *) 0, (char *) 0, _READ, 0 }, { 0,
(char *) 0, (char *) 0, _WRITE, 1 }, { 0, (char *) 0, (char *) 0, _WRITE
| _UNBUF, 2 } };

int fseek(FILE *fp, long offset, int origin) {
int result;
if (!(fp->flag & _UNBUF) && fp->base != NULL) {
if (fp->flag & _WRITE) {
if (fflush(fp))
return EOF;
} else if (fp->flag & _READ) {
if (origin == 1) {
if (offset >= 0 && offset <= fp->cnt) {
fp->cnt -= offset;
fp->ptr += offset;
fp->flag &= ~_EOF;
return 0;
} else
offset -= fp->cnt;
}
fp->cnt = 0;
fp->ptr = fp->base;
}
}
result = (lseek(fp->fd, offset, origin) < 0);
if (result == 0)
fp->flag &= ~_EOF;
return result;
}

int _flush(FILE *fp) {
return _flushbuf(EOF, fp);
}

int fflush(FILE *fp) {
int retVal = 0;
FILE *fpp = _iob;
if (fp != NULL) {
return _flush(fp);
} else {
while (fpp++ < _iob + OPEN_MAX)
if (_flush(fpp) == EOF)
retVal = EOF;
}
return retVal;
}
int fclose(FILE *fp) {
if (fp == NULL)
return EOF;
if (fp < _iob || fp >= _iob + OPEN_MAX)
return EOF;

_flush(fp);

if (fp->base != NULL)
free(fp->base);
fp->base = fp->ptr = NULL;
fp->cnt = 0;
fp->flag = 0;
close(fp->fd);
return 0;
}

int _fillbuf(FILE *fp) {
int bufsize;
if ((fp->flag & (_READ | _EOF | _ERR)) != _READ)
return EOF;
bufsize = (fp->flag & _UNBUF) ? 1 : BUFSIZE;
if (fp->base == NULL)
if ((fp->base = (char *) malloc(bufsize)) == NULL)
return EOF;
fp->ptr = fp->base;
fp->cnt = read(fp->fd, fp->ptr, bufsize);
if (--fp->cnt < 0) {
if (fp->cnt == -1)
fp->flag |= _EOF;
else
fp->flag |= _ERR;
fp->cnt = 0;
return EOF;
}
return (unsigned char) *fp->ptr++;
}

int _flushbuf(int c, FILE *fp) {
int bufsize;
int num_written;

if (fp < &_iob[0] || fp >= &_iob[OPEN_MAX])
return EOF;
if ((fp->flag & (_WRITE | _EOF | _ERR)) != _WRITE)
return EOF;
bufsize = (fp->flag & _UNBUF) ? 1 : BUFSIZE;

if (fp->base == NULL) {
if ((fp->base = (char *) malloc(bufsize)) == NULL) {
fp->flag |= _ERR;
return EOF;
}
} else {
num_written = fp->ptr - fp->base;
if (write(fp->fd, fp->base, num_written) != num_written) {
fp->flag |= _ERR;
return EOF;
}
}

fp->ptr = fp->base;
fp->cnt = bufsize;

if (--fp->cnt < 0) {
if (fp->cnt == -1)
fp->flag |= _EOF;
else
fp->flag |= _ERR;
return EOF;
}
if(c!=EOF)
*fp->ptr++ = c;
return c;
}

FILE *fopen(char *name, char *mode) {
int fd;
FILE *fp;
if (*mode != 'r' && *mode != 'w' && *mode != 'a')
return NULL;
for (fp = _iob; fp < _iob + OPEN_MAX; fp++)
if (fp->flag & (_READ ^ _WRITE))
break;
if (fp >= _iob + OPEN_MAX)
return NULL;

if (*mode == 'w')
fd = creat(name, PERMS);
else if (*mode == 'a') {
if ((fd = open(name, O_WRONLY, 0)) == -1)
fd = creat(name, PERMS);
lseek(fd, 0L, 2);
} else
fd = open(name, O_RDONLY, 0);
if (fd == -1)
return NULL;
fp->fd = fd;
fp->cnt = 0;
fp->base = NULL;
fp->ptr = NULL;
fp->flag = (*mode == 'r') ? _READ : _WRITE;
return fp;
}

int main(void) {
int c;
FILE *fp;

fp = fopen("/root/apu/cl/c8/test1.c", "r");
fseek(fp, 2, 0);
while ((c = getc(fp)) != EOF) {
if (putchar(c) == EOF)
exit(1);
}
fflush((FILE *) NULL);

return 0;
}